rammed earth retaining wall

planning a retaining wall, but I don't have a lot of experience. the only retaining walls I've built were made of either railroad ties or those silly cinder bricks with one round-ish face. neither of those will do.

the application: holding back raised garden beds that will surround an outdoor kitchen that's sunk maybe eighteen to twenty-four inches below grade. so the retaining wall will be about three feet tall. it will be under cover so it shouldn't get rained on, but it will definitely be in contact with moisture from the outside.

will rammed earth with a little cement mixed in work? how thick would something like that have to be? our dirt is pretty sandy, but there's a little bit of clay in it. friendlier alternatives to mix in instead of cement? would rammed earth finish up nicely with linseed oil like an earthen floor?

gabions have been suggested, but we don't have a lot of rocks, and I don't really care for the way gabions look.

my original plan was posts and wooden shoring with polyethylene to keep moisture from the wood, but I'm not a big fan of polyethylene, and that would mean buying a fair amount of lumber.

I would suggest Portland cement at about 5 to 7 percent although some like lime. You can test by seeing o=if it holds up in water in a week or so but I just did it and it was fine.

Portland cement will take a set in about an hour or so and holds up well in my experience. You are barely dampening it anyway. If it is muddy it is too wet. Just damp enough to stick together.

Some soils are fine with no cement.

The best mix is around 30 percent clay and 70 percent sand including any aggregate in the clay if it is hand dug. Too much aggregate and it will fall apart - too much clay and it may shrink excessively and crack.

For an exterior wall I don't think I would linseed it as you will want it to wick the moisture out. You could put gravel in a drainage trench under it for a French drain then a layer of plastic at the back to prevent moisture from getting into it, although I did not worry about it in my build. You would need a place for the water to drain to daylight at a lower level if there is much there.

- Glenn -

tel jetson

steward

Posts: 3395

Location: woodland, washington

82

posted 8 years ago

what about just using oil on the exterior side to keep moisture from wicking in?

Portland cement is a hydraulic cement, but Ernie is saying that hydraulic lime will work better with earth building than Portland cement which I think one site described as more harsh than hydraulic lime.

Hydraulic lime differs from hydrated lime in that it takes an initial set with water then continues to set from the CO2 in the air, from what I read.

I have had rammed earth walls stabilized with Portland cement up for about 4 years and they are still good and water resistant.

I haven't found the hydraulic lime here yet and asked around a bit today.

Portland cement may be easy to get but it does not blend well with earthen building IMO builders have noted failing systems and problems caused in several instances. hydraulic lime can cure under water and will not wick moisture over time. it also breathes like it is supposed to so its expansion and contraction rates are better.

i tend to be focused on earth building so this is the sort of stuff i keep an eye on.

the brand of hydraulic lime most folks use is Saint Astier. a bit pricey but there are other producers in the world. i dont know the names of them. a google search will bring many up for you to look at, most places in the country have at least one outfit that supplies it. you might check your local masonry supply folks, many of these places are set up for contractors so a call ahead will save you disappointment. have fun and good luck with your project.

finally getting ready to start on this project and I've got some more questions.

how should I prepare the ground under the rammed earth wall? ram it? gravel? this wall will be under cover, so drainage shouldn't be an issue.

the wall will be in the shape of an octagon. can I do one side at a time and move on to the next or will that create a weak point at each corner? I would really rather build forms for one side rather than eight sides, but if that's going to cause problems, I'll reconsider.

forms: does the working surface need to be prepared in any way to prevent sticking to the earth? something along the lines of petroleum jelly?

a little bit more information that might help folks answer my questions: this wall will be surrounding an outdoor kitchen that's sunk about two feet below grade. I'm planning to build the wall eight inches thick and 42 inches high. on the outside will be raised beds/hugel beds. the beds will rise eighteen inches to the top of the rammed earth wall and slope from there down to grade. I don't imagine any of that is going to put any huge amount of force on these walls, but if an engineer tells me otherwise, I'll probably have to defer.

thanks to those who've helped so far and those I'm hoping will help me out this time around.